In low bit rate video coding, the quantization of discrete cosine transform (DCT) coefficients produces well known artifacts in decoded images. The best known artifacts are the blocking effect and the ringing effect. Signal adaptive filters are generally used to remove these artifacts, while preserving details which belong to the image. Deblocking and deringing are two video post-processing techniques used to remove coding artifacts and improve the visual quality when rendering low bit rate coded video. The techniques used to achieve these tasks are computationally intensive and usually require high speed processors to be able to run in real time.
The blocking effect is grid noise along block boundaries and is mainly visible in smooth areas with low motion. The blocking effect is produced by the quantization of direct current (DC) coefficients. Usually deblocking filters try to remove the unwanted boundaries between adjacent blocks by low-pass filtering pixels on both sides of the block borders. However, this type of filtering may introduce undesirable blurring effects when applied to pixels which belong to real image edges. The ringing effect shows along object borders and is primarily due to the quantization of alternating current (AC) coefficients.